Conventionally, as a method for producing crystalline oxide materials, (1) a Horizontal Bridgman Method in which a temperature gradient is given horizontally to a growth chamber and liquefied materials in the growth chamber is solidified from a low-temperature seed crystal, (2) a Vertical Bridgman Method in which a temperature gradient is given vertically to a growth chamber, and liquefied materials in the growth chamber is solidified from a low-temperature seed crystal by moving the growth chamber, (3) a Vertical Gradient Freeze Method in which a temperature gradient is changed by fixing a growth chamber vertically and liquefied materials in the growth chamber solidified crystals from a low-temperature seed crystal (e.g., See Patent Document 1), have been known.
With reference to FIG. 1, the method for producing the crystals by the conventional Vertical Bridgman Method will be described. A seed crystal 4 and a raw material 2 are placed in a crucible 1. A raw material 2 is made into a liquefied raw material 2 by heating and liquefying with a heating element 6. Heating amount of the heating element 6 is regulated to retain crystal-producing furnace in an axial temperature distribution 5. The liquefied raw material 2 is cooled by moving the crucible 1 placed on a crucible support member 7 to a low temperature side. Then, the liquefied raw material 2 which have reached the crystallization temperature is grown to a crystal having the same crystal orientation as the seed crystal 4 and becomes a grown crystal 3.
The grown crystal 3 is then grown using the seed crystal 4 as nuclei, so that it can be grown as the grown crystal 3 having the same crystal orientation as the seed crystal 4.
Since an early phase of the crystallinity is transmitted to later stage of growth of the grown crystal 3, crystals grown in process of seeding is necessary to be maintained in high quality. The crystallinity depends on the state of an interface (solid-liquid interface) of the seed crystal 4 and the liquefied raw material 2 in the seeding process. Therefore, if temperature gradient in the vicinity of the crystallization temperature has a steep slope, the crystal rapidly grows. Then crystal structure and orientation cannot be inherited smoothly, and single-crystalline growth becomes difficult. Thermal stress due to a temperature variation is applied to the grown crystal 3 and a crystal lattice on solid-liquid interface distorts. A new crystal which reduces the distortion is grown at the solid-liquid interface and defects are increased in the crystal.
In the Vertical Bridgman Method, however, the temperature gradient in the vicinity of the solid-liquid interface is necessary so as to control the position of the solid-liquid interface. In order to grow single-crystalline crystal, crystal is produced by setting the temperature gradient necessary at the solid-liquid interface between the maximum temperature gradient that can maintain experimentally desired crystallinity and the minimum temperature gradient that can control the position of solid-liquid interface. This temperature gradient is reported as 8° C./cm at a constant diameter portion in the case of InP crystal (e.g., See Patent Document 2). In the conventional method, the temperature gradient in the vicinity of the solid-liquid interface is determined by the predetermined temperature of the heating element 4 for liquefying raw materials and the position of the crucible 1, while an accurate temperature regulation such as a local change in axial temperature distribution cannot be performed.
In the Vertical Bridgman Method, the solid-liquid interface in contact with a crucible wall generates stray crystals at the crucible wall. These stray crystals induce polycrystallization and cause degradation in crystallinity. In order to prevent this problem, temperature of solid-liquid interface of the crucible center can be lowered than temperature of the solid-liquid interface of the crucible wall, and the crystal growth at the solid-liquid interface can be progressed from center to wall of the crucible. In other words, the solid-liquid interface matched to an isothermal surface may be convexly formed toward the upper portion of the crucible. The convexly-formed solid-liquid interface can be achieved by removing heat selectively from the seed crystal 4.
The vicinity of the seed crystal 4 in the crucible 1 has a smaller capacity than the constant diameter portion and is affected by heat environment to increase in temperature variation. There is a concern that exceedingly high temperature may liquefy the seed crystal. Meanwhile, low temperature or precipitous temperature gradient may cause polycrystallization and crystal defect due to thermal stress.
The main object of the heat element used in the conventional method is to liquefy the raw material and to control the temperature gradient in the growing process at the constant diameter portion. However, there has been a problem that an accurate temperature gradient in the vicinity of the seed crystal in the seeding process cannot be controlled. According to the Patent Document 2, the temperature gradient in the vicinity of the seed crystal is reported as 60° C./cm in the case of InP crystal. This temperature gradient in the vicinity of the seed crystal controls the temperature gradient with the position of crucible and heat element for liquefying the raw material. Thus, the temperature gradient is used to avoid liquefying of the seed crystal in the case of temperature misalignment due to accuracy of placing crucible and temperature control of the heat element. However, in order to improve the crystallinity, as mentioned above, the growth of the seed crystal portion is also desired at the temperature gradient of 8° C./cm as used in the constant diameter portion.
For example, there may be a case that a raw material composition and a crystal composition differ each other and a crystal is grown from solution, such as K(Ta,Nb)O3 crystal. In this case, a crystal is grown after a raw material liquefied at sufficiently high temperature as 100° C. higher than the crystallization temperature in order to thoroughly decompose the raw material by giving soaking treatment. When crystal is grown without giving soaking treatment, deterioration of crystallinity and polycrystallization may occur. Therefore, in order to improve a process yield, it is necessary to give soaking treatment of the liquefied raw material before the growth. In the conventional method, however, the temperature gradient realizable in the vicinity of the seed crystal is 70° C./cm at a maximum and there has been a problem that realizing soaking temperature directly above the seed crystal exceeds the crystallization temperature of the seed crystal so that the seed crystal is dissolved.
A method to use a heat sink is known as a method to make precipitous temperature gradient in the vicinity of the seed crystal (e.g., See Patent Document 3). Primary object of the heat sink is to form the solid-liquid interface convexly toward the upper portion of the crucible in order to prevent polycrystallization due to a generation of stray crystals at the crucible wall. A carbon with high thermal conductivity is used in the heat sink to produce a crucible support member with larger diameter than a crucible diameter. The seed crystal is cooled by running a cooling pipe through the crucible support member. A method of using the heat sink provides a thermal insulation between the heat sink and the crucible so as to avoid excessive heat removal from the crucible to the heat sink in contact with the crucible. The precipitous temperature gradient in the vicinity of the seed crystal may be provided by cooling the seed crystal by means of the heat sink.
However, capacity of heat removal from the seed crystal is low due to the heat removal from the crucible support member with high heat capacity, and the temperature gradient in the vicinity of the seed crystal only up to 200° C./cm may be achieved. Refrigerant flow to run through the cooling pipe is increased in order to provide precipitous temperature gradient in the vicinity of the seed crystal. Then, the liquefied raw material may also be cooled through the thermal insulation and the seed crystal cannot be cooled locally. Therefore, there has been a problem that the misalignment in temperature due to accuracy of placing crucible and temperature control accuracy of the heat element cannot be reduced. There has been another problem such as cracks in the seed crystal due to exceeding temperature variation in a vertical direction of the seed crystal.
Patent Document 1: Japanese Patent Application Laying-Open No. 59-107996
Patent Document 2: U.S. Pat. No. 4,404,172 Specification, FIG. 3
Patent Document 3: U.S. Pat. No. 5,342,475 Specification